when you look in the feed & expansion tank, how deep is the mud in the bottom, and what colour is the water?

do you know when it last had inhibitor or cleaner added?

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The pipes come along the hall ground floor, from boiler ground floor.

The tank had about 3 gallons in it, with a brown covering of sludge about 1" on the bottom. I syphoned it out, then put cleaner in. Then I noticed the outlet is 5inches up, so the chemical is still in there. I have inhibitor for later. I doubt it has had inhibitor in for 30 years, if any. When I drained the water, it was black.

it is very possible that there is now a blockage in the feed and expansion pipe. Most usually it is where the 15mm joins onto the large circulating pipes. If you hold a magnet against this T-joint, is it attracted? If so it will be the black iron oxide sediment. Unless it is a compression joint that you can undo, you will have to cut out and replace the blocked section.

WOW! I bet you are correct. When I was syphoning the tank, the syphon kept blocking. There's a compression joint handy, on the silver downpipe, which might give you an idea of it's age. I predict a soaking in dirty water this morning.

First I scooped the chemical cleaner out out the expansion tank. Then tried to loosen one side of the compression fitting at 15mm to 28. It moved a little, then I realised I wasn't going to be able to tighten it as there is no room.

The outlet of the Expansion tank is 4 inches up, and there is a little extra thread poking out from the seal, so I soldered a length of 15mm to a fitting, screwed it onto the outlet, and made a little pump. Violent pumping and the blockage cleared. Then I added some garden hose to syphon it from the bucket.

All working, again with a new rad in the kitchen.

JohnD's prediction that there might be a leak was correct, and my prediction that I will get soaked was also correct, so happy days.